In one known method in press forming, press forming is performed on a blank material, which is a raw material for press forming having predetermined dimensions, in a state where a peripheral portion, which is not formed into a product shape part, is press-constrained by pressing the product shape part into the side of a die with a punch and thereby applying plastic deformation. In this case, the peripheral portion is press-constrained by sandwiching the peripheral portion of the blank material between the die of a press molding machine and a blank holder or between upper and lower blank holders. Hence, the peripheral portion is a so-called grip margin which is necessary for constraining the position of the blank material during forming, and is scrapped by being cut in a subsequent step. Therefore, it is advantageous to minimize the peripheral portion which corresponds to the grip margin described above so as to enhance the material yield.
On the other hand, in press forming, a forming method is proposed in which the sheet metal work is formed with a punch press while pretension is being applied to the peripheral portion of a sheet metal work (blank material) so as to pull the peripheral portion in a planar direction (see cited document 1). In this method, since the extension of the product shape part is increased by the pretension and the length of the sheet metal work in the shape of a flat plate before the press forming can be reduced, it is assumed that the material yield will be enhanced.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H09-150224